Optional reading
Checkouts are symlinks to other projects that have to be placed inside the project itself. When these links are place, the source of the projects they point to is used instead of the source being fetched from a repo.
Hotsauce as a matter of principle does not change anything in the projects or the project files.
Instead it keeps a list of hot (or cold) projects in ~/.hotsauce,
which it maintains through lein hotsauce subcommands.
Depending on the actual direct and indirect dependencies of the current
lein project, Hotsauce will adapt the CLASSPATH (and :cljsbuild :builds in case of ClojureScript)
according to which of these are hot and cold and whether Hotsauce is active
or not.
One of the ways Hotsauce differs from checkouts is that it can be used
without access to a matching repository version of the hot dependencies.
Actually hot dependencies do not even have to be installed in a repo.
Hotsauce emulates the CLASSPATH ordering (by shadowing :clsjbuild :builds ... :source-path), whereas cljsbuild makes its own attempt
at interpreting checkouts in a way meant to be compatible with standard lein.
I suspect that Hotsauce gives a result that is actually more like standard lein - but to be honest, I'm not sure.